Intel tweaks EM64T for full AMD64 compatibility

Adds missing instructions to Pentium 4 core

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IDF Intel is preparing to update its Pentium 4 core, the better make its 64-bit processors more compatible with AMD's 64-bit processors, company documents seen by The Register reveal.

The new core, dubbed 'G-1', will replace the current 'E-0' core in Q4. According to a note sent to Intel customers, samples of the updated processors will ship on 17 October, with full shipments commencing just under a month later, on 14 November.

The tweak, which will require a BIOS update, adds a pair of instructions to the EM64T instruction set: LAHF and SAHF. These two codes copy content back and forth between the chip's status flags and its AH register.

Register readers may recall reports last year, published not long after Intel announced EM64T, that the technology lacked certain instructions found in the AMD64 instruction set. That's right, there were two: LAHF and SAHF.

At the time, Microprocessor Report analysts speculated that this had happened because Intel based EM64T on a version of AMD's AMD64 documentation written down before AMD incorporated LAHF and SAHF into its own instruction set.

Intel has always admitted EM64T isn't 100 per cent compatible with AMD64, but it has maintained that the incompatibility is negligible. It has presumably decided now that sufficient quantities of software out there use LAHF and SAHF, so it needs to take them on board.

Hence the G-1 update for the P4, and presumably we'll soon see comparable updates for EM64T-equipped Xeon processors - perhaps the two missing instructions will be introduced with 'Paxville', the 90nm dual-core Xeon MP and DP core due to ship in Q4. So far, only the P4 521, 531, 541 and 551 are mentioned in the core-upgrade notice, but it's likely the 6xx series will be updated too, along with the Pentium D and Pentium Extreme Edition. ®